It is known that the reflector antennas which are generally used with microwave frequencies often employ aligned structures in which the primary feed is positioned in front of the reflector. Double-reflector Cassegrain systems are generally aligned with the auxiliary reflector positioned on the axis of the main reflector. However, in such structures the primary feed or the auxiliary reflector is so positioned that they form obstacles to radiation from or to the main reflector, producing what is known as a masking effect which results principally in a reduction in the gain of the antenna and an increase in the level of the side lobes. To overcome this drawback, use is sometimes made with Cassegrain structures of the method known as rotating the plane of polarization involving utilization of an auxiliary reflector composed of parallel wires or plates which are transparent to one polarization. However, this method is only economical when applied to small reflectors and in principle is compatible only with linear polarization. With circular polarization it would be necessary to use an external polarizer covering the whole of the antenna. To avoid the masking effect, ancillary reflectors are often employed whose primary feed is offset from the radiation zone of the main reflector, the latter generally possessing a plane of symmetry. Nevertheless, this technique still has one disadvantage; even when illuminated by an emitter of waves with pure polarization, such as a Huygens source, the reflector produces a cross-polarization radiation pattern which is antisymmetrical about the plane of symmetry. With circular polarization this phenomenon becomes apparent as an angular error in the direction of the main lobe, an increase in the ellipticity level of the radiated wave, and an asymmetry in the changes in the latter level relative to the plane of symmetry of the antenna.
Besides the losses of gain which result from them, these phenomena also cause a reduction in the rejection factor for rain echoes in radars employing circular polarization and a deterioration in the accuracy and angular stability of tracking radars.
To alleviate these problems, which are related to the curvature of the reflector, it is necessary to increase the focal length of the latter. Apart from the resultant increase in size, this procedure calls for high-gain primary feeds and the spillover from these at the periphery of the reflector gives rise to undesirable interference radiation, false echoes, and sensitivity to jamming sources or to terrestrial heat radiation in the case of a space telecommunication antenna.